My main drop out time is in the evenings between 5ish and 9ish. After communication between myself Spotify and my internet provider, the blame was laid at BTs door and bandwidth throttling. I have got round that issue by going to the premium service and I download what I want to listen to first.
Sometimes Spotify just cuts out and you will see certain tracks going red and announcements appear that that music is no longer available in this country. As Spotify continually updates what music is available and what is licenced to play where this can cause temporary issues.
There have also been problems with Spotify and Mcafee, particularly when Mcafee decided Spotify was a virus and everyone got cut off. Though I have not heard of issue since, I have experienced some drop outs as my new firewall/virus protector does scans and updates.
It just goes a bit wonky sometimes, it is computer based after all!